1. Field of the Invention (Technical Field)
The present invention relates generally to desalination of brine, and, more particularly, to the use of solar energy to drive a reverse osmosis plant.
2. Background Art
The global need for high quality water is acute. A serious obstacle to fresh water production is the energy cost. Solar energy is free, but devices that harness that energy can be expensive, and the use of the energy is usually inefficient. A solar still, for example, requires over 500 calories of heat to distill one gram of water.
The present invention, called Solar Energy Desalination System (SEDS), provides a way to desalinate seawater or brackish water economically. It operates optimally in areas of abundant sunshine.
Most desalination methods that use solar energy utilize the heat of the sun to provide the thermal energy to evaporate seawater in a flash distillation plant. For example, U.S. Pat. No. 5,672,250 teaches such a method. U.S. Pat. Nos. 4,504,362 and 5,645,693 teach focusing sunlight with mirrors to achieve higher temperature for boiling seawater.
U.S. Pat. No. 5,507,943 shows a method of desalinating seawater using wave motion to oscillate a float attached to a lever arm that drives a piston to pump seawater into a reverse osmosis (RO) device. It uses solar energy indirectly, since the waves are driven by sun-powered wind.
U.S. Pat. No. 5,004,535 shows the use of a solar photovoltaic power source that drives a motor and pump to force water into a reverse osmosis unit.
U.S. Pat. Nos. 3,558,242, 4,367,140, 5,193,988, 5,306,428, 5,462,414, 5,797,429, and 6,017,200 employ piston means to recover some of the energy of the rejected brine. The pistons are driven by external sources of power.
U.S. Pat. No. 4,883,823 shows a system that can use solar energy to boil a low-boiling-point working fluid to drive pistons that pump seawater into a reverse osmosis desalination unit. When the high-pressure working fluid vapor drives a piston for this type of application, it does so at essentially constant pressure. When the exhaust valve opens, the vapor exits the cylinder in a free expansion with little drop in temperature. Since the vapor is still hot, its heat energy can be used to preheat the liquid feed to the boiler in a recuperator (heat exchanger). Patent 4,883,823 and similar prior art do not show such a recuperator, resulting in lower efficiency of the system. The present invention does incorporate a recuperator.
The present invention also incorporates a novel motorless boiler feed pump. U.S. Pat. No. 5,685,152 presents a solar power system which contains a motorless boiler feed pump. One of the disadvantages of this design is that the condenser must be located above the boiler in order for the liquid flow from the condenser to gravity flow into an intermediate tank and then gravity flow into the boiler. Another disadvantage is that the intermediate tank must be cooled in order for the cooled liquid to flow into it without vaporizing vigorously. Then when the valve is opened to allow the liquid to flow into the boiler, high-pressure vapor from the boiler rushes into the intermediate tank and condenses on the cool walls. This represents a significant loss of energy.
In one embodiment of the present invention, a particular type of xe2x80x9cmotor, expander, compressor, or hydraulicsxe2x80x9d system, hereinafter referred to as xe2x80x9cMECHxe2x80x9d, is incorporated in the apparatus. The MECH apparatus and functions are disclosed in copending U.S. patent application Ser. No. 09/715,751, entitled xe2x80x9cApparatus Using Oscillating Rotating Pistonsxe2x80x9d filed on Nov. 16, 2000, the disclosure of which is herein incorporated.
Various objects, advantages and novel features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention is directed to a desalination system. A solar powered boiler outputs a pressurized vapor to drive an expander that generates output motive force. A pump is responsive to the motive force to output pressurized seawater or brackish water. A reverse osmosis unit receives the pressurized seawater to output fresh water and pressurized brine. In a particular embodiment, a hydraulic motor receives the pressurized brine and outputs an augmenting motive force to the pump. A heat recuperator transfers heat from expander exhaust gas to boiler feed liquid. In another embodiment, the boiler feed liquid is pumped by a novel motorless feed pump.